A lubrication oil pressure at a certain level is necessary when an internal combustion engine is running. Without sufficient lubrication wear for instance of bearings or even seizure of the engine will be the result. When starting a vehicle equipped with an internal combustion engine as the only power source a lubrication oil pressure is built up within a couple of seconds, i.e. there is a sufficient oil pressure prior to when the driver normally requires power output from the engine.
For a plug-in hybrid electric vehicle the electric vehicle traction motor will drive the vehicle as long as power is available from the battery and the request for power is moderate. However, at some point, either when the power runs low on the battery or if the driver requests more power than what the electric vehicle traction motor can produce, the internal combustion engine will have to start. Especially, if the driver wants to accelerate the vehicle at an already relatively high rpm on the powertrain, the delay in the response will be significant if the internal combustion engine is to first have a sufficient lubrication oil pressure. Thus, the problem is to as fast as possible get power from when starting the internal combustion engine without damaging it.